1. Field of the Invention
The present invention relates to optical print heads and to means for transmitting light emitted from the print heads to a photosensitive surface, and more particularly to means for accurately aligning the transmitting means and the print head to maintain high resolution and good exposure uniformity.
2. Description of the Prior Art
High speed optical printing devices used in data processing systems are intended for converting electric input signals into printed form, and include photosensitive means and light sources such as linear arrays of light emitting diodes (LED's), photodiodes, or similar devices. Light from a source is often transmitted to the photosensitive means via a plurality of gradient index optical fibers forming an imaging lens array. Such imaging lens arrays are commercially available as SELFOC (a trademark of Nippon Sheet Glass Co., Ltd.) lenses in a staggered two-row bundle of optical fibers as described in U.S. Pat. No. 3,658,407, patented on Apr. 25, 1972. Reference may be made to that patent for details of the lens arrays.
A gradient index optical fiber 10 is illustrated in FIG. 1. The refractive index distribution of the optical fiber varies parabolically outwards from the longitudinal axis of the optical fiber towards the peripheral portion thereof.
FIG. 2 shows an imaging lens array 12 formed of a staggered, two-row bundle of optical fibers 10. If the distances L.sub.1 and L.sub.2 are properly selected and equal, then there is obtained an erect image at a magnification of one of an object. Distance L.sub.TC represents the total conjugate of the lens (i.e., the distance between the object and image planes). Accordingly: EQU L.sub.1 =L.sub.2, and EQU L.sub.TC =L.sub.F +L.sub.1 +L.sub.2
Where L.sub.F is the length of optical fibers 10 of imaging lens array 12.
Imaging lens arrays such as shown in FIG. 2 have been proposed for transmitting light between an array of LED's 14 and photosensitive means such as a photoconductive member 16. For optimum resolution, dimensions L.sub.TC, L.sub.1, and L.sub.2 must be accurately set and maintained. In the design of prior art devices such as shown in U.S. Pat. No. 4,147,412, it is assumed that the total conjugate L.sub.TC does not vary from lens to lens and that, as such, the spacing between the object plane and the image plane can be fixed at the total conjugate distance. While this approximation is adequate for low resolution applications, in fact however, the lens parameters L.sub.TC and L.sub.F vary considerably between manufacturers, and even between lenses of the same manufacturer.
LED's generate a considerable amount of heat during operation, and the heat is absorbed by both the print head and the lens array, which commonly have different rates of thermal expansion. Rigid mounts for the lens array on the print head result in excessive stress due to the different expansion rates.
It is, therefore, an object of the present invention to provide a mount for locating an imaging lens on a print head in a way which allows for convenient and accurate compensation for variations in lens array dimensions, while accommodating the different rates of thermal expansion of the imaging lens array and the print head.